Quantification of gas-accessible microporosity in metal-organic framework glasses

Abstract

Metal-organic framework (MOF) glasses are a new class of glass materials with immense potential for applications ranging from gas separation to optics and solid electrolytes. Due to the inherent difficulty to determine the atomistic structure of amorphous glasses, the intrinsic structural porosity of MOF glasses is only poorly understood. Here, we investigate the porosity features (pore size and pore limiting diameter) of a series of prototypical MOF glass formers from the family of zeolitic imidazolate frameworks (ZIFs) and their corresponding glasses. CO2 sorption at 195 K allows quantifying the microporosity of these materials in their crystalline and glassy states, also providing excess to the micropore volume and the apparent density of the ZIF glasses. Additional hydrocarbon sorption data together with X-ray total scattering experiments prove that the porosity features of the ZIF glasses depend on the types of organic linkers. This allows formulating design principles for a targeted tuning of the intrinsic microporosity of MOF glasses. These principles are counterintuitive and contrary to those established for crystalline MOFs but show similarities to strategies previously developed for porous polymers.

Precise control over gas transporting channels in zeolitic imidazolate framework glasses

“…2D), which shows that size selectivity of the adsorption increases. This again is contradicting to prior findings, where the pore diameters and thereby apertures get significantly larger in a g ZIF-62 (20,34,37), whereas other data provide the hypothesis that short-range order deformation makes pores in a g ZIF-62 inaccessible (18,38). The CO 2 diffusivity in this case is two orders of magnitude slower than this of the neat ZIF-62, which we attribute to a collapsing of the porechannels.…”

“…Envelope density of a g ZIF-62 nP of 1.3469 g/cm 3 , is in good accordance with the recently discovered data, which was determined as 1.35 g/cm 3 from CO 2 sorption experiments (37). Envelope and skeletal densities of the crystalline ZIF-62 were found to be 1.4178 g/cm 3 and 1.4728 g/cm 3 respectively, being noticeably higher than the calculated crystallographic density from the same work (1.29 g/cm 3 ) (37). We suppose that such a contradiction might be caused by the difference between simulated crystalline matrix and real object.…”

Modulating Liquid–Liquid Transitions and Glass Formation in Zeolitic Imidazolate Frameworks by Decoration with Electron-Withdrawing Cyano Groups

“…We speculate that the bim − -based secondary linkers are too bulky to allow for an exothermal framework collapse, as it is documented that the pore space in the crystalline glass formers ZIF-62 (containing bim − ) and TIF-4 (containing mbim − ) is significantly smaller than the pore space in ZIF-4. 34 In this work, we implement smaller Xim − linkers, exhibiting strongly electron-withdrawing cyano groups 35 directly bound to the imidazolate core (Figure 1), in ZIF-4 and study their influence on framework melting and glass formation as well as the materials' porosity. A series of crystalline ZIF-4 derivatives termed ZIF-4-CN x (composition Zn(im) 2−x (CNim) x , CNim − = 4-cyanoimidazolate, 0.04 ≤ x ≤ 0.39) and ZIF-4-dCN x (composition Zn(im) 2−x (dCNim) x , dCNim − = 4,5-dicyanoimidazolate, 0.10 ≤ x ≤ 0.28) was synthesized by either solvothermal or mechanochemical methods.…”

De-solvation of metal complexes to construct metal-organic framework glasses

“…The 2D structure of 11c was also confirmed by its gate-opening gas sorption behavior. To our knowledge, these pore size fine-tuning and dynamic structural changes upon gas inclusions haven't been observed in reported MOF glasses (42)(43)(44).…”